1. Field of the Invention
The present invention relates to an optical pickup servo control apparatus for forcing an optical pickup to track a medium, on which information is recorded, in an optical disk unit, an optical card reader, or the like while retaining a specified positional relationship. More particularly, the present invention relates to a servo control apparatus capable of adjusting, for each optical pickup, the individual differences in characteristics of a servo control apparatus resulting from individual errors in optical pickups caused during manufacturing.
2. Description of the Related Art
An apparatus using an optical pickup includes an optical disk unit and an optical card reader. Hereafter, the present invention will be described using the optical disk unit as an example.
In an optical disk unit, an optical disk is rotated about a rotation axis by a spindle motor. This causes an optical pickup to move in the radial direction of the optical disk and align with the optical disk. Then, the optical pickup reads or writes information from or on the optical disk.
The optical pickup routes a light beam generated by a semiconductor laser serving as a light source to an objective lens via a known optical system. Then, the objective lens shrinks the light beam to provide a spotlight of a very small diameter, and irradiates the spotlight onto the optical disk. Then, the light reflected from the optical disk is routed to the optical system via the objective lens. Consequently, a light receiver in the optical system provides a light received signal associated with a change in the reflected light.
In this kind of optical disk unit, numerous tracks and pits are formed at intervals of several microns in the radial direction of an optical disk. To record or read information on or from the optical disk, a light beam must be forced to follow a track or pits while the state of focusing providing a beam spot of 1 micron or less in diameter is being retained. However, an optical disk may become eccentric or swell. Slight eccentricity of an optical disk displaces a focusing position of a light beam, and swelling thereof deviates a focal point of a beam spot. Therefore, the beam spot cannot be irradiated onto a track of the optical disks as it is. To solve this problem, a servo control apparatus is used to perform servo control so that an optical pickup will move on a track or pits while maintaining the focal point providing a beam spot of 1 xcexcm or less. Specifically, a servo control apparatus for an optical pickup performs two kinds of servo control; that is, focus servo control for controlling a focal point of a light beam and tracking control for moving a beam spot to follow a track or pits.
In efforts to achieve the foregoing servo control, a focus actuator (focus coil) for moving an objective lens of an optical pickup perpendicularly to an optical disk in order to vary a focal point, and a tracking actuator (track coil) for moving the objective lens in the radial direction of the optical disk in order to vary an irradiation point in the tracking direction are provided. An optical system includes, for example, four elements making up a four-division light receiver, and is designed to provide a focus error signal FES associated with a deviation of the focal point of a beam spot and a tracking error signal TES associated with a displacement of the beam spot from a tracing position by processing the outputs of these elements. A focus servo control or a component of a servo control apparatus for an optical pickup processes a light received signal output by the light receiver, generates a focus error signal, and feeds back the focus error signal to the focus actuator so as to control the focus actuator. A tracking servo control processes the light received signal, generates a tracking error signal, and feeds back the tracking error signal to the tracking actuator so as to control the tracking actuator.
In the foregoing focus servo control or tracking servo control, an offset occurs due to individual variations in characteristics of each part of the optical pickup or to an error in mounting each part. Therefore, the focus does not always coincide with a recording surface of an optical disk when a focus error signal has a zero level. For a similar reason, the gain of the servo control system varies. Therefore, the focus servo control and tracking servo control are required to control their offsets and gains for each optical pickup.
In an optical pickup servo control apparatus, differential amplifiers are employed as means that use a light received signal to generate a focus error signal and a tracking error signal respectively. In the past, a bias resistor and a feedback resistor connected to these differential amplifiers have been realized with variable resistors, or a variable resistor has been connected to an offset control terminal. Thus, offsets and gains have been controlled. Furthermore, the variable resistors are adjusted in a process of manufacturing an optical pickup so that offsets will be zero and gains will be specified values.
However, a resistance of a variable resistor having a sliding portion tends to vary due to an environmental change or over time. Therefore, an offset and a gain, which have already been controlled, may vary as environments change or time passes. This results in unsatisfactory servo control.
In efforts to solve the aforesaid problems, the present applicant has disclosed an apparatus in Japanese Unexamined Patent Publication (Kokai) No. 62-222438 and Japanese Unexamined Patent Publication (Kokai) No. 62-141644. In the disclosed apparatus, a detector for detecting an offset value and a compensating means that compensates for the detected offset value are included in the focus servo control and in the tracking servo control respectively in an optical pickup, whereby an offset is automatically controlled. Japanese Unexamined Patent Publications (Kokai) Nos. 1-125733, 2-294940, 3-152722 and 4-19833 also disclose similar apparatuses each having a detector for detecting an offset value and a compensating means that compensates for the detected offset value.
The aforesaid kind of servo control apparatus is unsusceptible to an environmental change or a time-sequential change because no variable resistor is used. However, the servo control apparatus poses the following problems:
(1) Since a focus servo system or a tracking servo system is adjusted during normal operation of an optical disk unit, the normal operation is slowed down by time required for adjustment.
(2) For automatic adjustment, a peak detector (envelope detector), an A/D converter, and other extra circuits are needed. This leads to an increase in the scale of circuitry. Eventually, the optical disk unit becomes larger and costs increase.
When it comes to gain control, in Japanese Unexamined Patent Publication (Kokai) No. 63-224034, the present applicant has disclosed a technology allowing the tracking servo control to perform gain control in such a manner that permits stable servo control irrelevant of a change in the shape of a track groove. This technology can control only a gain variation resulting from the shape of a groove on an optical disk. The technology, therefore, does not cope with variations in servo gain resulting from other factors, such as, performance of an actuator, and assembly precision of an optical system.
The aforesaid prior art has not dealt with control of a focus servo gain.
For the reasons mentioned above, gain control is currently achieved using a variable resistor.
When a variable resistor is used to control an offset and a gain occurring in a servo control apparatus for an optical pickup, a controlled state is subject to change. This disables stable servo control. When an offset value detector is installed in the apparatus to compensate for an offset, the operating speed decreases, the size expands, and the cost increases.
An object of the present invention is to realize a servo control apparatus of an optical pickup in which an offset and a gain can be adjustable for each servo control apparatus, and influences from changes in environmental conditions and changes with the passage of time are reduced without a drop in operation speed, an increase in size and an increase in cost.
A servo control apparatus of an optical pickup according to the present invention includes a nonvolatile memory and a compensation portion. The nonvolatile memory stores compensatory values corresponding to differences of servo characteristics of each servo control apparatus, and these compensatory values of servo characteristics are measured with the assistance of external measurement instruments when this servo control apparatus is manufactured at a factory and so forth. The compensation portion compensates an optical pickup servo signal according to the compensatory values stored in the nonvolatile memory.
A servo control apparatus of an optical pickup according to the present invention is a focus servo control apparatus or a tracking servo control apparatus, or includes these two focus and tracking servo apparatuses.
Compensatory values of offset and gain of servo characteristics are stored in the nonvolatile memory. If a compensatory value of the gain is stored in the nonvolatile memory, the compensation portion includes a gain adjustable amplifier. If a compensatory value of the offset is stored in the nonvolatile memory, the compensation portion includes a summing circuit.
A compensatory value setting method of a servo control apparatus according to the present invention includes a step for measuring a signal of a portion of said servo control apparatus by external measurement means, a step for changing a compensatory values until said signal of said portion reaches a predetermined condition, and a step for storing said compensatory values into said nonvolatile memory.